Heat storage water tank

ABSTRACT

A heat storage tank suitable for solar house heating systems. The tank includes a rigidly supported bottom panel covered with a layer of stones of substantially uniform size which are bonded together thermally and mechanically at their points of contact with each other, but which have voids forming air passages remaining between them, bottom insulation below said bottom panel, a bottom air passage between the bottom insulation and the bottom panel, and at least one opening in the bottom panel whereby air can pass from said bottom air passage into said layer of stones.

Where heat is produced by a variable source such as terrestrial sunlightand used to maintain the temperature of a variable heat load such as ahouse, it is essential to provide some means of storing heat. A tank ofwater is frequently used for this purpose, because water is readilyavailable and has a high heat capacity. Sunlight adds heat to the water,raising its temperature, and the load withdraws heat from the water,lowering its temperature. In simple systems for house heating, the tankwater temperature must stay above the desired inside temperature of thehouse. At the other extreme, many practical problems with tank materialsand structures arise unless the tank water temperature is kept below theboiling point (212° F. at sea level), and for many materials, 180° F. isa top limit. The amount of water required for storing a given amount ofheat is inversely proportional to the temperature range of the water inthe tank.

Not all the water in a tank is at the same temperature. Warmer waterrises to the top and cooler water settles to the bottom, because thewarmer water expands and becomes less dense. Water itself is a fairlygood insulator, so that heat flow, in the absence of mass flow, is slow.A tank in which hotter water stands at the top and cooler water at thebottom is said to be stratified.

In solar heating systems, a fluid is passed in succession through asun-heated collector where it is heated by absorbed sunlight; and a heatexchanger where it warms the water in the heat storage tank. "Used"fluid issuing from the heat exchanger returns to the collector forre-heating. If the fluid is water, it may circulate directly through thetank, making a separate heat exchanger unnecessary. Other heat transferfluids, such as air, oil, or ethylene glycol, require a suitable heattransfer structure to keep them from mixing physically with the tankwater.

Since the solar heat collector is in a cold ambient environment, itloses heat at a rate in proportion to the average temperature of thefluid being heated in it. Maximum efficiency of heat collection is,therefore, promoted by keeping the temperature of the "used" fluidentering the collector as low as possible. This lowest possible tanktemperature is approximately equal to the desired house heatingtemperature.

Delivery of heat from the storage tank to the house takes placecontinuously, and thus is at a much lower peak rate than delivery ofheat to the tank from intermittently operated solar collectors. In mosthouse heating systems, heat is finally delivered by warmed air. This airmust be warmed at least to the temperature of the room, but if a largeair flow rate is provided, it need not be warmed much above roomtemperature to deliver the heat required to keep the room warm. If alarge exchanger is provided to transmit heat from the storage tank waterto the circulating room air, the required amount of heat can betransmitted by tank water only slightly above room temperature.

Thus, the water at the bottom of the storage tank should be kept closeabove room temperature, while the water in the remainder of the tankshould be stratified with the top layer at the maximum storagetemperature, extending to a depth corresponding to the amount of heatstored.

A tank designed to store heat over an entire year must be very wellinsulated to prevent excessive losses to its surroundings. It is wellknown that the amount of heat collector area required by a house heatingsystem which stores heat for an entire year is less than half thecollector area required for short-term storage systems.

Thus, it is the primary object of this invention to provide a long-termheat storage tank construction and control system which will produceefficient collection, storage, and delivery of solar heat energy.

A more specific object is to provide a practical, well-insulated hotwater storage tank having a large air-water heat exchanger on its lowersurface.

A second specific object is to provide a means for controlling thetemperature of the water layer in contact with the heat exchanger.

A third specific object is to provide a means for controlling thecirculation of fluid through solar collectors so that the storage tankwater temperature is sharply stratified with maximum storage temperatureat the top, while the collectors are operated at the lowest practicaltemperature.

Another object of the invention is to provide a heat storage tanksuitable for solar house heating systems, including: a rigidly supportedbottom panel covered with a layer of stones of substantially uniformsize which are bonded together thermally and mechanically at theirpoints of contact with each other, but which have voids forming airpassages remaining between them, bottom insulation below said bottompanel, a bottom air passage between said bottom insulation and saidbottom panel, and at least one opening in said bottom panel whereby aircan pass from said bottom air passage into said layer of stones; rigidlybraced side panels having an inner layer of outside insulation and anouter layer of outside insulation and a continuous side air passagebetween them, communicating with the surface of said layer of stones,and with a plenum chamber near the top of said side panels, whereby airmay pass from said bottom air passage through said layer of stones,through said side air passage to said plenum chamber; a liningimpervious to water having a bottom, sides, and a top, and topinsulation above said top; means for transferring heat from a solar heatcollector to water contained in said tank, and heat delivery fluid meansfor delivering heat from said tank; means for circulating water from acontrollable height within said tank to the inside bottom surface ofsaid tank, and means for constraining said circulating water to flowover the inside bottom tank surface whereby the temperature of saidinside bottom surface is kept substantially equal to the temperature ofwater at said controllable height; and control means responsive to thetemperature of said heat delivery fluid for adjusting said controllableheight, and for adjusting the rate of said means for transferring heatfrom a solar heat collector responsive to said controllable height andto the temperature at said solar collectors.

It will be apparent to those skilled in the art that these objects andothers are achieved by the construction and control systems now to bedescribed in some detail with the help of the accompanying drawings.

FIG. 1 is a general section view of an insulated tank and heat exchangeraccording to this invention, showing means for supplying heated waterfrom a solar collector and means for delivering heat to a house from thetank.

FIGS. 2 and 2A are a section of an insulated tank and heat exchangershowing air means for transferring heat from an air-heating solarcollector to the tank.

FIGS. 3, 4, and 5, are graphs showing water temperature plotted againstdepth in the tank for various conditions of operation.

FIG. 6 is a graph of heat transfer within the heat exchanger related tothickness of the heat transfer layer and length of the air path.

FIGS. 7 and 8 are simplified circuit diagrams for the control circuitsshown schematically in FIGS. 1 and 2.

Turning to FIG. 1, the tank is nested in a concrete pit 10. Shortvertical wooden posts or legs 11, support a bottom platform 12, whichmay be of plywood. On top of the platform is a layer 13, several inchesthick of stones, graded for size, bonded together with cement, whichform a heat exchanger as taught in copending U.S. Pat. application, Ser.No. 618,502, filed on Oct. 1st, 1975. Side panels 14 are braced from thewalls of the pit by horizontal wood posts 15. The tank has a durable,waterproof liner 16, which may be fabricated of butyl rubber. The tankis substantially full of water.

Layers of fiberglass insulation, 17, 18 and 19, are disposed around thelegs 11, and braces 15. A horizontal feed air passage 20, is providedbetween the platform 12, and the insulation layer 17. Vertical airpassages 21, are provided between the insulation layers 18 and 19,jacketing the sides of the tank.

The top edge of the tank is closed by panels 22, which create a plenumpassage 23, at the top of the air passages 21. A waterproof, durableflexible cover 24, which may be of fabric reinforced butyl rubber issecured to the top edge of the tank. It floats on the water in the tank,supporting a thick layer of fiberglass insulation 25.

Heat may be brought to the tank by a piping system and a pump whichcirculates water from the tank through any of the many known designs ofsolar heat collector. This piping system is shown with an inlet 26, andan inlet 26, and an outlet 27, both near the bottom of the tank. Waterdrawn into the inlet is forced by pump 28, through collector 29,draining back into the tank through piping outlet 27.

Heat is delivered from the tank by an air circulating system. Air drawnfrom the house H is forced into the passage 20, by a blower 30 and duct31. It issues from passage 20, through an opening 32, which may be aslot, a series of holes, or a grating, into the grouted gravel layer 13,where it flows outward toward the sides of the tank, absorbing heat fromthe bottom of the tank. From the gravel layer, the air flows into thepassages 21, and upward to the plenum passage 23. In passage 21, the airflow forms a sheath around the tank, defining the temperature at thatpassage approximately at the temperature of the house, and thus limitingthe rate of heat flow to the concrete walls 10, regardless of thetemperature of the water inside the tank.

By natural convection, the coolest water in the tank will be at thebottom, where it is drawn into the inlet 26 of the piping to thecollector. Warmed water delivered from the collector rises in the tank,where it becomes stratified according to temperature.

In the absence of provision to bring heat to the bottom of the tank,heat will flow out into the circulating house air until the tank bottomis too cold to provide enough heat. Therefore, this invention includes awater-circulating pump 33, which runs continuously, a flexible inlethose 34, and a weighted horizontal barrier membrane 35, just above thebottom of the tank. By means of a servomotor 36, responsive totemperature at the sensor 37, in the air duct 31, the inlet opening 38is slowly automatically raised or lowered in the tank to find the waterstratum of the desired temperature, and this water is spread over thebottom under the confining barrier 35.

To make full use of the heat storage capacity of this tank, it isnecessary to store heat at the maximum temperature which the tankmaterials can endure. On the other hand, when heat is being deliveredfaster than it is being stored, it should be collected at temperaturesonly a little above use temperatures. Limit switch 39, which senseswhether the circulation inlet 38 is at the bottom of the tank tellswhether the tank is gaining or losing heat. If the inlet 38 is anywhereabove the bottom of the tank, net heat is flowing out, but if it is atthe bottom, net heat is flowing in for storage. In the first case,maximum circulation through the collector is called for, but in thesecond, circulation should be slowed down to produce the highesttolerable temperature as measured at the collector 29, by sensor 40.

The means shown for slowing down the water flow to achieve hightemperatures is a smaller pump 41, connected in parallel with the pump28. Check valves 42 and 43 in series with the pumps prevent back flowwhen only one pump is running. Under optimum sunlight and high outdoortemperature, the top tank temperature is reached with full speed of pump41. The variable speed motor driving pump 41 is slowed to keep thetemperature at sensor 40, up when sunlight is less or outdoortemperature is lower.

When the circulating pump inlet 38 is not at the bottom of the tank,pump 28 runs and pump 41 is shut off as long as the temperature at thesensor 40 is above the bottom tank temperature.

When pipe inlet 38 is up, net heat is flowing out and switch 39activates large pump 28 for maximum circulation in the collector andpump 41 does not run.

When pipe inlet 38 is down, net heat is flowing in and switch 39activates small pump 41 and large pump 28 does not run. The speed ofpump 41 is controlled by temperature sensor 40.

The circuit performing this function is illustrated in more detail inFIG. 7.

FIGS. 2 and 2A show essentially the same tank as FIG. 1, except that anair-heating solar heat collector is used instead of a water-heatingcollector. The same grouted gravel heat exchanger which is used fordelivering heat to the house is now also used for absorbing heat fromair coming from the solar heat collector. The duct system to the solarheat collector 42, includes an inlet duct 43, connected to the plenum23, and an outlet duct 44, connected to the air passage 20. As long asthe circulator input 38, is not at the bottom of the tank, a blower 45,drives heat transfer air through the collector loop whenever thetemperature at sensor 46, is above minimum tank temperature. If thecirculator input is at the bottom of the tank, a smaller parallel blower47, operates at controlled speed to supply heat at maximum tank storagetemperature.

The circuit performing this function is illustrated in more detail inFIG. 8.

There are many possible known control circuits for accomplishing thecontrol functions which have been described. The particular simplifiedcircuits illustrated in FIGS. 7 and 8 make use of variable resistancetemperature sensors generally known as RTD's.

In FIG. 7, the three wire reversible A.C. motor 36, FIG. 1, is activatedin response to the temperature at sensor 37 in such a way that inlet 38,is moved upward when the temperature at 37 is below a preset temperatureand downward when it is above that preset temperature. A Wheatstonebridge circuit consisting of the RTD 37, resistors 53 and 54, and anadjustable rheostat 55, is energized from a D.C. source 52. Thedifference voltage from the bridge works through amplifier 56 toenergize relay 57, which connects motor 36 to run upward or downward inresponse to the temperature at 37.

In FIG. 8, the temperature sensor at 46, FIG. 2A, controls motors 45 and47. RTD 46 and resistors 58, 59 and 60 in series, and 61 form aWheatstone bridge energized by D.C. source 62, to produce a positivesignal when the temperature at 46 exceeds a predetermined minimum usefultemperature. If the cam-actuated switch 39 shows the inlet 38 to beabove the bottom of the tank this positive signal is amplified at 63,energizing relay 64 to turn on motor 45, causing rapid circulation ofheat-transfer fluid through the collector.

If the cam actuated switch 39 shows the inlet 38 to be at the bottom ofthe tank, amplifier 63 becomes inoperative, but the power amplifier 65becomes responsive to the potential at the junction between resistors 59and 60. It drives motor 47 at a speed responsive to the temperature at46. When the temperature approaches a predetermined upper safe limit,corresponding to the choice of resistors 58, 59, 60 and 61, the motor 47runs at maximum speed. As the temperature falls below that limit, themotor speed also falls rapidly. Thus the temperature of the circulatingheat transfer fluid, which rises when the circulation rate falls, iskept near the maximum temperature which the tank can endure.

It will be evident that the same circuits can be used with theembodiment of FIG. 1, if only sensor 40 replaces sensor 46, and themotors of pumps 28 and 41, replace motors 45 and 47.

FIGS. 3, 4 and 5, show temperature distributions in the tank for variousconditions of operation. Temperature T is plotted on a horizontal scaleand height H above the bottom of the tank on a vertical scale.

FIG. 3 shows the condition when the tank contains little stored heat andmore heat is being taken from the bottom every day than the collectorsupplies. The warmest water is at the top, and a growing depth of thecoldest water starts at 48, just above the confined layer which deliversheat to the house air. The temperature in the confined layer at thebottom is maintained by circulating water brought down from 49.

FIG. 4 shows the condition when there is a substantial amount of heatstored, and heat is coming from the collector faster than it is beingdelivered. Now the cold water layer above 48 is thinner but circulatingwarmer water still is brought down from 50. The point 50 is movingdownward as water heated by the collector increases the warmed layerbetween 50 and 51, at the expense of the cold layer between 48 and 50.

FIG. 5 pictures a temperature distribution when heat is coming in fasterthan it is being delivered. The confined layer is now the coldest waterin the tank, and circulating water of suitable temperature can be takenfrom 48. Water going to the collector from 48 returns hot, rising to thetop of the tank. This increases the layer of maximum temperature waterat the expense of the coldest water at the bottom.

Thus, it is seen that the temperature of the collector is always kept aslow as possible consistent with storage of heat at the highesttemperature the tank can tolerate. When heat is being delivered, thecollector receives water at the lowest tank temperature and returns itonly a little warmer than house air. When heat is being stored, thecollector first warms all the cold water in the tank to usabletemperature. Then it takes in the coolest remaining water and deliversit at maximum tank temperature. In this way, maximum collectorefficiency and maximum heat storage capacity are both assured.

The curves of FIG. 6 are theoretical curves showing how much heattransfer can be expected in a grouted gravel bed having a giventhickness, length of air path, width of air path, heat conductivity, andair flow speed. In English units, let t be the bed thickness, l be thebed length, and w be the bed width, all in feet. Let d be the stonediameter inches, v be the air speed in feet per second, and k be theheat conductivity of the stone bed in BTU/hr ft °F. Let T₁ be thetemperature of entering air and T₂ be the temperature of water at thetank bottom, in degrees Fahrenheit.

The solution of the differential equation for heat flow in the rock bedcan be expressed in terms of a dimensionless length ξ=l/β and adimensionless thickness θ=t/√αβ. In English units, the characteristiclength β is approximately 0.287 υφ ft., while the characteristicthickness √αβ is about 0.148 (kd)^(1/2) ft.

The solution can be expressed in a triple summation over the indices n,m, and s, where m is restricted to odd integers and s, to even integersless than m. Q is the rate heat flows into the tank, BTU/hr. ##EQU1##

The notation S! means S factorial.

The curves in FIG. 6, for three particular values of the dimensionlessrock bed thickness θ, are numerically computed from this formula. Twosimplifying assumptions have been made, that net heat conduction of theair in the thickness direction is negligible and that heat conductionthrough the rock in the length direction is negligible. These curvesagree qualitatively with experimental results.

The usefulness of these curves is that they enable a person skilled inthe art to estimate the heat transfer effectiveness of a rock bed havinga given stone size, length, width, thickness, rock conductivity, andheat transfer air speed. For example, it can be deduced from FIG. 6 thatthe heat flow increases with total area of the stones, but is less thanproportional to that area. Thus, FIG. 6 provides a means of testingwhether a given rock bed design will have adequate heat transfercapacity and of determining how economic it will be.

Having described a heat storage tank which will store heat efficientlyfor long periods of time and a control system suitable for efficientcollection, storage and delivery of solar heat energy, and havingexplained the design sufficiently to be useful to those skilled in theart,

What we claim is the following:
 1. A heat storage tank suitable forsolar house heating systems having a solar heat collector, including:arigidly supported bottom panel covered with a layer of stones ofsubstantially uniform size which are bonded together thermally andmechanically at their points of contact with each other, but which havevoids forming air passages remaining between them, bottom insulationbelow said bottom panel, a bottom air passage between said bottominsulation and said bottom panel, and at least one opening in saidbottom panel whereby air can pass from said bottom air passage into saidlayer of stones; rigidly braced side panels having an inner layer ofoutside insulation and an outer layer of outside insulation and acontinuous side air passage between them, communicating with the surfaceof said layer of stones and a plenum chamber near the top of said sidepanels, whereby air may pass from said bottom air passage through saidlayer of stones, through said side air passage to said plenum chamber; alining impervious to water having a bottom, sides and a top, topinsulation mounted above said lining top; means connected fortransferring heat from a heat source to water contained in said tank,and heat delivery fluid means for delivery of heat from said tank; meansfor circulating water from a controllable height within said tank to theinside bottom surface of said tank, and means for constraining saidcirculating water to flow over the inside bottom surface of said tank,whereby the temperature of said tank inside bottom surface is keptsubstantially equal to the temperature of water at said controllableheight; and control means responsive to the temperature of said heatdelivery fluid for adjusting said controllable height, and for adjustingthe rate of said means for transferring heat from the solar heatcollector responsive to said controllable height and to the temperatureat said solar collector.
 2. A heat storage tank suitable for solar househeating systems according to claim 1, in which said means fortransferring heat from the solar collector includes a piping system anda pump for drawing water from near the bottom of said tank, circulatingit through said collector and returning it near the bottom of said tank.3. A heat storage tank suitable for solar house heating systemsaccording to claim 1, in which said means for transferring heat from asolar collector includes a duct system and a blower for drawing air fromsaid plenum, circulating it through said solar heat collector andreturning it to said bottom air passage.
 4. A heat storage tank suitablefor solar house heating systems according to claim 2, in which saidcontrol means controls the means for transferring heat from the solarcollector so that,water is circulated through said collector by saidpump only when the temperature of said collector is above a minimumuseful temperature, water for circulating to said collector issubstantially the coldest water in said tank, when the coldest water insaid tank is below a useful temperature, water is circulated to saidcollector at a maximum rate, and when the coldest water in said tank isat least at useful temperature, water is circulated to said collector ata reduced rate, returning to said tank at the highest temperaturepermissable in said tank.
 5. A heat storage tank suitable for solarhouse heating systems according to claim 3, in which said control meanscontrols the means for transferring heat from a solar collector sothat,air is circulated through said collector by said blower only whenthe temperature of said collector is above a minimum useful temperature,when the coldest water in said tank is below a useful temperature, airis circulated to said collector at a maximum rate, and when the coldestwater in said tank is at least at useful temperature, air is circulatedto said collector at a reduced rate, returning to said tank at thehighest temperature permissable in said tank.
 6. A heat storage tanksuitable for solar house heating systems, including:a rigidly supportedbottom panel, a heat exchanger mounted under said bottom panel, a bottomair passage connected to said heat exchanger, rigidly braced side panelsconnected to said bottom panel having an inner layer of outsideinsulation and an outer layer of outside insulation, a continuous sideair passage between them, a plenum chamber near the top of said sidepanels communicating with said air passage whereby air may pass fromsaid bottom air passage through said heat exchanger, through said sideair passage to said plenum chamber; means connected for transferringheat from a heat source to water contained in said tank, and heatdeliver fluid means for delivery of heat from said tank, means connectedto said tank for circulating water from a controllable height withinsaid tank to the inside bottom surface of said tank, and means in saidtank for constraining said circulating water to flow over the insidebottom surface of said tank, whereby the temperature of said tank insidebottom surface is kept substantially equal to the temperature of waterat said controllable height. .Iadd.
 7. In a liquid filled heat storagetank, a heat exchanger, means movable in height connected inside thetank and connected to the heat exchanger for drawing liquid from acontrollable height within said tank, passing said liquid through saidheat exchanger for delivering heat to use, and means connected to theliquid drawing means for returning said liquid to said heat storage tankat the bottom of said tank, whereby the temperature of said heatexchanger is kept substantially equal to the temperature of said liquidat said controllable height. .Iaddend. .Iadd.
 8. In a liquid-filled heatstorage tank according to claim 7, a solar collector, means connectedfor drawing said liquid from the bottom of said tank and circulatingsaid liquid through said solar collector and returning said liquid tosaid tank. .Iaddend..Iadd.
 9. A liquid-filled heat storage tankaccording to claim 8 and control means for limiting the rate of flow ofsaid liquid to said collector so that liquid returned from said solarcollector to said tank is at maximum allowable storage temperature..Iaddend..Iadd.
 10. A liquid-filled heat storage tank according to claim7 wherein said heat exchanger is the bottom of said tank and the liquiddrawn from said controllable height is constrained beneath a barriermembrane to flow over the area of said bottom of said tank beforeescaping to the upper side of said membrane. .Iaddend. .Iadd.
 11. Aliquid-filled heat storage tank according to claim 10 and means fordrawing said liquid from the bottom of said tank, circulating saidliquid through a solar collector and returning said liquid at maximumallowable storage temperature to said tank. .Iaddend.